1. Field of the Invention
This invention relates to a web transport device for transporting a web wound on a shaft on the supply side to a shaft on the take-up side, and in particular to a device for controlling the braking of the web.
2. Description of the Prior Art
In a searching device for effecting an information search by the use of a microfilm on which marks are provided for locating a desired image on the film, the film is drawn out from a cartridge containing therein a supply reel on which the microfilm is wound in the form of a roll, the leading end of the film is wound onto a take-up reel and the take-up reel is rotatively driven by a motor while the supply reel is fiollow-up-driven, whereby the film is fed from the cartridge to the take-up reel (this will hereinafter be referred to as forward feed), and the supply reel is rotatively driven by a motor while the take-up reel is follow-up-driven, whereby the film is fed from the take-up reel to the cartridge (this will hereinafter be referred to as reverse feed).
In such a searching device, to locate a frame of the film in which a desired image is recorded, the film is fed at a high speed in the forward feed direction or in the reverse feed direction, marks on the film are optically detected by a mark detector, the mark detection signals put out by the mark detector when it detects the marks are counted by a counter, and when the count value agrees with the address number of the desired frame, a brake is applied to the rotary shaft of a reel on the drive side (the take-up side) and the rotary shaft of a reel on the driven side (the supply side) at a time to stop the film, and the frame which has advanced too much from a predetermined projecting position is returned at a low speed and is stopped at the projecting position.
However, in the above-described high-speed searching, as shown in FIG. 1 of the accompanying drawings, when a film having a total of about 3000 frames has been fed at a high speed from the shaft on the supply side on which the film is wound to the shaft on the take-up side, the number of revolutions per minute N.sub.1 of the rotary shaft of the reel on the drive side which takes up the film is almost constant from the point of time whereat the feeding is started till the point of time at which the feeding of the film of 3000 frames is terminated, but the number of revolutions per minute N.sub.2 of the rotary shaft of the reel on the driven side which supplies the film is varied about the number of revolutions per minute N.sub.1 of the drive side shaft by the variation in the diameter of the film roll on the reel with the vicinity of 1500 frames (the point of time at which one half of the length of the film has been fed) as the boundary. Also, the slack of the film when suddenly stopped from the high speed is represented by curve .delta.. That is, from the point of time at which the feeding has been started (0 frame) till the point of time at which 1500 frames have been fed, the film is stopped while being subjected to tension and the slack of the film increases remarkably from the vicinity of the point of time at which the number of fed frames exceeds 1500 frames. In the case of a film having a different number of frames (a different full length of film), the number of frames in the vicinity of the point of time at which the slack occurs will differ.
This phenomenon occurs due to the differences in angular speed and inertia force between the shafts on the drive side and the driven side when a brake is applied thereto. This may lead to a serious problem that if the amount of slack .delta. is smaller than the amount of slack formed by the back-lash of the shaft hole of the reel in the film cartridge on the driven side and the diameter of the film roll on the driven side, the film will vibrate due to the back-lash and, if the marks vibrate on the mark detector, erroneous counting will occur and the desired frame cannot be located.
Therefore, it has been attempted to effect independently the film control with the take-up reel side as the drive side and the control with the supply reel side as the drive side, but the back-lashes of the mechanical inertia system on the supply reel side, the inertia system on the take-up reel side, the shafts and the supply and take-up reels cannot be made equal to one another, and in a different form of cartridge, the difference therebetween is great and therefore, so that controlling stoppage of the film is complicated and difficult.